1. Field of the Invention
The present invention relates to an anti-reflection film which is arranged in order to prevent external light from reflecting on a window or a surface of display devices etc. Specifically, the present invention relates to an anti-reflection film applied on a surface of a liquid crystal display (LCD), CRT display, organic electroluminescent display (ELD), plasma display (PDP), surface-conduction electron-emitter display (SED) and field emission display (FED) etc. Among these, this invention relates to an anti-reflection film applied on a surface of an LCD, especially a transmission type LCD.
2. Description of the Related Art
In general, displays are used under external light whether they are used indoors or outdoors. The external light incident to a display surface is reflected on the surface so that a displayed image is interfered with by the reflected image and the quality of display decreases. Hence, it is necessary to provide a display surface with an anti-reflection function, and further, improvements of the anti-reflection function along with introductions of other extra useful functions are being demanded.
In general, an anti-reflection function is realized by forming an anti-reflection layer having a multilayer structure with repeating high refractive index layers and low refractive index layers made of a transparent material such as metal oxide on a transparent substrate. The anti-reflection layer having this type of multilayer structure can be obtained by a dry coating method such as chemical vapor deposition (CVD) and physical vapor deposition (PVD). In the case where the anti-reflection layer is formed by a dry coating, while there is an advantage of fine thickness controllability, there is also a problem of low productivity due to a limitation of a deposition process performed in a vacuum chamber, which is unsuitable for mass production. Thus, wet coating methods, which make it possible to provide a large display, produce continuously, and reduce costs, attract attention as a forming method of an anti-reflection layer
In addition, an anti-reflection film in which the anti-reflection layer is arranged on a transparent substrate generally has a hard coat layer formed by curing an acrylic material between the transparent substrate and the anti-reflection layer for the purpose of providing a surface hardness to a relatively soft surface of the anti-reflection layer. The hard coat layer is provided with a high level of surface hardness, abrasion resistance, luster and transparency due to the acrylic material.
The hard coat layer made by curing an acrylic material, however, is liable to take charge because of its insulation quality and has a problem of attracting dust to the surface of the anti-reflection film in which the hard coat layer is arranged. Thus, it is required that the anti-reflection film has an antistatic function. In the case where an anti-reflection film in which a hard coat layer and an anti-reflection layer are included is arranged on a surface of an LCD, it is required that the anti-reflection film has an antistatic function in order to prevent a charged surface from adversely affecting the inner circuitry of the LCD.
In order to provide an antistatic function to an anti-reflection film having a hard coat layer and an anti-reflection layer, a method of adding conductive agent to the hard coat layer or a method in which an antistatic layer is arranged between the substrate and the hard coat layer or between the hard coat layer and the anti-reflection layer can be used.
<Patent document 1>JP-A-2005-202389
<Patent document 2>JP-A-2005-199707
<Patent document 3>JP-A-H11-092750
<Patent document 4>JP-A-2007-121993
<Patent document 5>JP-A-2005-144849
<Patent document 6>JP-A-2006-159415
It is possible to reduce reflection of external light by arranging an anti-reflection film having a hard coat layer and anti-reflection layer on the surface of a display device and utilizing its anti-reflection function so that a contrast in the light place can be improved. In addition, since the transmittance of the anti-reflection film can be improved, it is possible to display an image more brightly. The anti-reflection film is also expected to have an energy saving effect due to a reduction of the power consumption of the backlight etc.
In the case where an antistatic function is provided to the anti-reflection film by adding conductive materials, the optical properties of the resultant anti-reflection film depend on the types of the conductive materials added thereto. In the case where conductive materials are added to the anti-reflection film, a problem of an insufficient contrast occurs due to a fall in luminance during display of a white image (this luminance is referred to as white luminance) when the film is arranged on an LCD surface.
In the case of a method in which an antistatic function is provided by newly arranging an antistatic layer, the production cost becomes high due to an increase in the number of layers resulting in a fall in productivity. In addition, since it is necessary in general to arrange an antistatic layer with high refractive index between the layers, color generation or color unevenness tends to occur by the antistatic layer. Especially in the case where the antistatic layer and the low refractive index layer (the anti-reflection layer) are formed by a wet coating method, there was a problem of color unevenness of the anti-reflection film according to the unevenness in layer thickness.
The anti-reflection film in which an antistatic function is provided by adding conductive materials to the hard coat layer requires lower production costs than the anti-reflection film in which an antistatic function is provided by newly arranging an antistatic layer. However, the former type of anti-reflection film requires a large amount of conductive material and leads to problems such as increase in cost of material, decrease in hardness of the hard coat layer, decrease in visible light transmittance, and interference fringe generation caused by the difference in refractive index between the substrate and the hard coat layer etc.